1. Field of the Invention
This invention relates to frequency division multiplexing for multi-drop networking which permit multiple devices to communicate with a central master of a common medium. More specifically this invention relates to Slotted Synchronous frequency division multiplexing when the channel is divided into a number of sub-channels which are synchronized.
2. Related Art
Multi-drop networking has traditionally used TDMA techniques to allow multiple devices to communicate with a central master over a common medium. FDM has also been used for this purpose.
U.S. Pat. No. 5,680,388m to Kahre, which is incorporated herein by reference. Kahre summarizes a number of ways of multiplexing. Similarly, multiple carrier wave channels are allocated by multiple access by frequency division multiplexing with mobile units which involves transmitting and receiving being divided into time gaps. Kahre describes several techniques for performing multiplexing such as TDMA, FDMA and CDMA.
Orthogonal Frequency Division Multiplexing OFDM has been used to multiplex frequencies, into groups or bands. Typical of such systems are those shown in U.S. Pat. No. 5,682,376 to Hayashino, et al. which is incorporated herein by reference. Hayashino discloses a complex multiplier that complex-multiplies a carrier modulation signal group for decoding the phases and amplitudes of a plurality of carriers which are orthogonal to each other in the frequency axis by a complex signal group having a predetermined specific pattern which varies in phase at random. An Inverse Fourier transformer performs Inverse Fourier transformation on an output of the complex multiplier, for transforming a digital signal which is multiplexed on the frequency axis to an OFDM signal on the time axis.
In U.S. Pat. No. 5,809,030 to Mestdagh, et al., A time domain multiplexing system is described which utilizes a number of frequencies to transmit data in a transmission system and applies delay to synchronize the data. Alternatively, the addition of a cyclical prefix is described to synchronize the data. However, Mestdagh notes there is a penalty in the system. Where the carriers used by the different transmitters are generated by different carrier generators, their frequencies will not be perfectly equidistant. Accordingly the subsets of carriers in Mestdagh do not constitute a perfectly orthogonal set of carriers. This, as pointed out in Mestdagh, results in a frequency interference effect due to this non-orthogonality. Mestdagh accepts this penalty as necessary in the present transmission system.
In local systems, such techniques also have certain drawbacks, primarily in terms of required processing power or capacity. In a local system it is difficult to justify high capacity processing at each station as pointed out in Mestdagh which increases exponentially with the number of inputs. Accordingly, where a low cost expectation is required such as with multiple telephone sets at a signal location, it is not practical to invest complex modulation and demodulation techniques in telephone sets.
The current invention provides efficient modulation at a master station and allows great flexibility in client or slave stations from simple two bit/symbol single-channel modulation and demodulation to multi-channel multiple-bit modulation which may be incorporated into low cost limited complexity units.
The present invention also permits dynamic selection of the frequency band to use for transmission, thus dealing effectively with channel nulls and no channel equalization is required.
The present invention is a useful technique for short-distance moderate-rate multi-drop networking, such as multi-channel home-telephone for Voice over IP(VoIP).
The present invention permits varying degrees of complexity, from a simple form of QAM modulation to full-DMT, all in the same overall configuration. This is possible because of the excess prefix equalization and echo-cancellation simplicity.